That might seem like a simple question, maybe even a stupid one. Yet, anyone who says they know the rulebook definition of swing in Major League Baseball is lying. Because there isn’t one.

The word “swing” appears in the 2018 edition of the Official Baseball Rules 21 times: 15 times in the text, six times in the index. None of those appearances include the definition of a swing. “Checked Swing” is a bold-faced term in the index, but it refers only to the comment on Rule 8.02(c) concerning when it is acceptable for managers to appeal an umpire’s decision. That comment contains eight mentions of a “half swing” — a half swing being one situation when a manager may request that the home plate umpire ask a colleague for help — but none of those mentions define a half swing, nor state explicitly if a half swing on a ball outside the strike zone should be ruled a swinging strike or a successfully checked swing resulting in a ball. The...